client principal selection and UI

[Sam Hartman]

There are a few things that I forgot to mention.  Frist, you will note
that my proposal completely ignores the concept of a default
principal.  The proposal is focused on an environment wher eyou have a
lot of different services and realms and it is not clear you have a
meaningful default principal.  I consider the question of deciding
when default principals are useful and how they should work to be
open.  I don't have good answers.

The other big open issue is how you get default hinting.  For example
you probably want some hinting so that you don't have to get prompted
for the first time you talk to each service in your own local domain.

I'll note that other models are possible that are a lot closer to what
we have today.

--Sam